Thermoplastic polyamides are frequently used in the form of glass-fiber-reinforced molding compounds as construction materials for components that are exposed to high temperatures during their service life, ending with thermooxidative damage. The occurrence of thermooxidative damage, which is manifested, for example, in a decline of the mechanical characteristics, can be delayed through the addition of known heat stabilizers, but it cannot be permanently prevented. Improvement of the heat aging stability of polyamides is absolutely desirable, since through this longer service lives for thermally stressed components can be achieved or the risk of failure thereof can be reduced. Alternatively, improved heat aging stability can also enable the use of components at higher temperatures.
The heat aging stability in the known polyamide molding compounds, especially over longer periods of exposure to heat and at temperatures higher than 200° C., is still insufficient.
For improvement of the processability of semiaromatic polyamides of type proposed, PA9T, US 2003/0023008 A1 proposes the addition of specific polyamides having a carbon/amide ratio of 7-12 in a concentration of 10-50 wt %. The glass transition and crystallization temperatures of PA9T are said to be clearly reduced, while the melt temperature is retained, so that lower mold temperatures and thus conventional heating of the injection molds are enabled. As Comparison Example CE5 confirms, the blend of PA9T with 20 wt % PA6, however, is not seen to be advantageous. Furthermore, no improved heat stability over the semiaromatic polyamide is shown.
WO 2006/74934 A1 concerns heat-stabilized molten compounds based on a polyamide mixture and a transition metal compound, which can be used, for example, in the engine region of automobiles or in the electric/electronics field. The polyamide mixture consists of at least 50 wt % of a semicrystalline or amorphous polyamide (a1), the melting point or glass transition point of which is at least 200° C., and a second, semicrystalline or amorphous polyamide (a2) having a maximum C/N ratio of 7, the melting or glass transition point of which lies at least 20° C. below that of (a1). The preferred transition metal is iron, which is used as iron oxide in the examples. The preferred polyamide (a2) is PA6. In the examples only the aliphatic polyamides PA46 and PA46/6 are used as blending components in combination with 25 wt % PA6. In addition, only the combination of copper- and iron stabilization shows an improvement of the heat resistance. As the Comparison Examples A-E clearly show, the addition of PA6 or PA11 to PA46 does not produce any improvement of the heat stability over a conventional copper-stabilized PA46.
US 2009/0127740 A1 describes a method for producing a component that consists of at least two parts that adhere to each other, where at least one part is made of a blend of semiaromatic and aliphatic amide. In the most preferred variant here, 65-95 wt % semiaromatic polyamide (A) is mixed with 5-35 wt % aliphatic polyamide (B) in order to improve the adhesion of thermoplastically overmolded parts produced on the basis of semiaromatic polyamide molding compounds. PA6 is also mentioned, among others, as aliphatic polyamide and incorporated in Example 1, but where specifically modified polyolefins must be added. Component (A) is composed of equal parts by weight PA 6T/DT and PA 6T/66 (D=2-methylpentanediamine). Nothing is reported about an improvement of the heat stability of these molding compounds.